1. Field of the Invention
The present invention relates to shaft member coupling structure for lockably coupling two shaft members in a condition in which they coaxially extend with a coupling part in the center. More specifically, the invention relates to shaft member coupling structure which can be applied to a spool valve forming a clutch valve of a hydrostatic continuously variable transmission.
2. Description of Background Art
Various configurations of a hydrostatic continuously variable transmission are known. For example, hydrostatic continuously variable transmissions have been proposed and disclosed in JP-A No. H6-42446; JP Patent No. 2920772; JP-A No. H9-100909 and JP-A No. 2005-256979 by the applicants. These hydrostatic continuously variable transmissions disclosed in these patents and applications are each provided with a swash plate type plunger pump, a swash plate type plunger motor and a hydraulic closed circuit that connects a discharge port and a suction port of the swash plate type plunger pump to a suction port and a discharge port of the swash plate type plunger motor. The hydrostatic continuously variable transmission is configured so that a pump swash plate is driven by an engine, a pump cylinder and a motor cylinder are connected and are arranged on an output shaft, the rotation of a motor swash plate is regulated, and an angle of the motor swash plate can be variably adjusted.
A hydrostatic continuously variable transmission configured as described above is known wherein a clutch valve that connects and cuts off an oil passage on the high pressure side and an oil passage on the low pressure side respectively forming the hydraulic closed circuit is provided. A quantity in which the rotational driving force from the hydraulic pump is transmitted to the hydraulic motor is controlled and clutch control that cuts off this rotational transmission is executed. For example, in JP-A No. 2005-256979, an automatic clutch using such a clutch valve is disclosed. This clutch valve is provided with a valve spool movably arranged in a spool hole axially extending in the supporting shaft that rotatably supports the hydraulic pump and the hydraulic motor, and connects and cuts off the oil passage on the high pressure side and the oil passage on the low pressure side by axially moving the valve spool. The clutch valve is provided with a spring (energizing means) that energizes the valve spool in a direction of disengagement and a centrifugal governor that generates force corresponding to the input revolution speed, is opened and closed according to the balance among energizing force by the spring, governor force and a load depending upon internal pressure (high pressure), and executes control for connecting and cutting off the oil passage on the high pressure side and the oil passage on the low pressure side.
In the above-mentioned clutch valve, as the valve spool requires a part that receives the energizing force by the spring and governor force, a part that guides to enable axial smooth movement in the spool hole and a part that connects and cuts off the oil passage on the high pressure side and the oil passage on the low pressure side according to the axial movement, the valve spool is formed in an axially long shape. In this case, as high precision is required for the peripheral dimension of a guide part fitted into a guide hole formed in the supporting shaft and guided to be axially moved in the spool hole and the peripheral dimension of a valve part fitted to a part in which the oil passage on the high pressure side and the oil passage on the low pressure side are open in the spool hole for connecting and cutting off the oil passage on the high pressure side and the oil passage on the low pressure side according to the axial movement, the above-mentioned clutch valve has a problem that the manufacture of the valve spool is difficult and a great deal of manufacturing cost is required. In addition, when the precision is not met, the operational performance may be deteriorated.
The inventors proposed that the valve spool be formed by coupling a first spool member provided with a part for forming the guide part and a second spool member provided with a part for forming the valve part based upon the above-mentioned problems. Further, the inventors proposed a configuration wherein the first spool member and the second spool member coaxially extend and are mutually lockably coupled by a coupling pin extending in a direction perpendicular to the axis as shaft member coupling structure. However, in this configuration, the coupling pin is merely fitted into coupling holes extending in the direction perpendicular to the axis in the first and second spool members and a problem wherein the coupling pin falls away and may hinder rocking occurs. To prevent this problem, it is conceivable that the coupling pin is press-fitted into the coupling holes. However, a problem occurs wherein a press-fitted part may be broken during the press fitting and a problem occurs in that the man-hours needed for press fitting increases.